Method for heat recovery from synthesis gas

ABSTRACT

An improved method and apparatus for cooling and recovering heat from soot-containing hot gases obtained by the partial combustion of hydrocarbons is disclosed. The improvement comprises partly cooling the hot gases by flowing them through one or more straight tubes of at least two meters in length at a mass velocity of at least 100 kg/m2/sec. The gases are subsequently further cooled in one or more helically coiled tubes connected to the straight tubes. The invention is particularly suitable for generating high pressure steam from hot gases obtained at moderate as well as high pressures.

Unite States Patent ter Haar et al.

Jan. 23, 1973 METHOD FOR HEAT RECOVERY FROM SYNTHESIS GAS Inventors:Leonard W. ter Hear; Johan P. Schungel, both of The Hague, NetherlandsAssignee: Shell Oil Co., New York, N.Y.

Filed: Nov. 6, 1970 Appl. No.: 87,451

Foreign Application Priority Data Nov. 11, 1969 Netherlands ..6916941US. Cl. ..l65/163, 122/7 R Int. Cl ..F28d 7/10 Field of Search..l65/l63,159; 122/24, 7 R

References Cited UNITED STATES PATENTS 1/196] Hofstede et al ..l22/32 XFOREIGN PATENTS OR APPLICATIONS 634,687 1/1962 Canada ..l22/7 PrimaryExaminer-John J. Camby Assistant ExaminerW. C. Anderson Attorney-Glen R.Grunewald and John H. Colvin [57] ABSTRACT An improved method andapparatus for cooling and recovering heat from soot-containing hot gasesobtained by the partial combustion of hydrocarbons is disclosed. Theimprovement comprises partly cooling the hot gases by flowing themthrough one or more straight tubes of at least two meters in length at amass velocity of at least 100 kg/m /sec. The gases are subsequentlyfurther cooled in one or more helically coiled tubes connected to thestraight tubes. The invention is particularly suitable for generatinghigh pressure steam from hot gases obtained at moderate as well as highpressures.

4 Claims, 3 Drawing Figures PATENTEDJAH23|975 3.712.371 sum 1 F 2 FIG. 3

INVENTORSZ LEONARD W. TER HAAR JOHAN P. SCHUNGEL THEIR ATTORNEYPATENTEDJM123 I973 3,712,371

SHEET 2 [1F 2 la H FIG. 2

INVENTORSI LEONARD W. TER HAAR JOHAN P. S'CHUNGEL 1% A MW THEIR ATTORNEYMETHOD FOR HEAT RECOVERY FROM SYNTHESIS GAS BACKGROUND OF THEINVENTION 1. Field of the invention This invention relates to animproved method and apparatus for cooling and abstracting heat fromgases which have extremely high temperatures and which contain mattersubject to deposition in heat exchanger tubes through which the gasesflow. This invention is particularly applicable to the generation ofhigh pressure steam, e.g., steam having a pressure of 50l50 atmospheres,using the sensible heat from gases obtained by the partial combustion ofhydrocarbons with oxygen or oxygen-enriched air, e.g., synthesis gasobtained from a pressure oil gasification process.

2. Description of the Prior Art Crude synthesis gas produced by thepartial combustion of hydrocarbons generally is discharged from thereactor at a temperature of from 1,300 to 1,400C or higher, thus makingit an obvious source of potential energy. The thermal energy insynthesis gas, however, can be recovered only with great difficultyutilizing conventional heat exchangers, because of the presence in suchgases of large amounts of soot (i.e., free carbon), often up to 5percent or more, which tends to deposit on the inside of heat exchangertubes. U.S. Pat. No. 2,967,515 to Hofstede et al. describes a means ofsubstantially overcoming the problem of soot deposition by the use ofhelically coiled cooling tubes which are disclosed as being considerablyless subject to deposit formation than straight cooling tubes.

While effective in overcoming the soot deposition problem, the use ofhelically coiled tubes places certain other limitations on the process,particularly in respect to permissible pipewall temperatures and thepressure differential between the cooling medium and the gases to becooled. These limitations result from the lower mechanical strength ofhelically coiled tubes due to their method of manufacture. (Generallycoiled tubes are formed by winding straight tubes which results inunroundness which in turn appreciably reduces the mechanical strength ofthe coiled tube.) Because of this decreased strength, helically coiledtubes are not wellsuited for the generation steam at high pressures,e.g., 50 to 150 atmospheres or higher, from hot gases obtained atmoderate pressures. Under such conditions, the pressure of the coolanton the outside of the coiled tube considerably exceeds that of the hotgases flowing through the tube. Moreover, high tubewall temperatures areoften experienced which also contribute to tube failures.

This problem cannot be overcome merely by reducing the velocity of thegases flowing in the helical tube. Such a reduction in velocity, whilepossibly decreasing tubewall temperatures because of reduced heattransmission, will also result in correspondingly lower steam pressuresand in an increased risk of soot deposition on the inside wall of thetubes. Once a thin layer of soot has deposited on the wall of thecooling tube, a further decrease in heat transmission is experiencedresulting in still lower steam pressures and an undesirable increase inthe discharge temperature of the gas. The method and apparatus hereinprovided substantially overcomes the aforementioned problems.

SUMMARY or THE INVENTION It has now been found that helically coiledtubes can be safely and effectively used for the cooling of hightemperature soot-containing gases with the concomitant generation ofhigh pressure steam, if the gases prior to being passed through thecoiled tubes are first partly cooled by flowing them through one or morestraight tubes under the critical conditions hereinafter described.Thus, in accordance with the invention, high temperature soot-containinggases obtained by the partial combustion of hydrocarbons are flowedthrough one or more straight tubes, the outsides of which are in contactwith a coolant, preferably water, at a mass velocity of at leastkilograms/meter /second (kg/m /sec.). The length of the tube andvelocity are selected so the gases passing through the straight tube arecooled to a temperature not exceeding 1,200C. Preferably, thetemperature of the gases discharging from the straight tube will bebetween 1,200 and l,000C. The gases are subsequently further cooled,e.g., to a final temperature of about 200-400C, by flowing them throughone or more helically coiled tubes which are also in contact with thecoolant and which are connected to the straight tubes.

It has been found that by maintaining the mass velocity of the gases atleast 100 kglm lsec, soot deposits which normally form more rapidly instraight cooling tubes than in helical tubes, occur to a surprisinglysmall extent and do not interfer with the operation of the process aswould be expected. The upper limit of the mass velocity of the gases isgoverned primarily by permissible tubewall temperatures. Preferably,mass velocities of above 500 kglm lsec are avoided since at these highvelocities the temperature of the tubewalls become so high thatresistance to the erosive effect of soot particles rapidly diminishes.Hence the mass velocity of the gases in the straight tube should be from100 to about 500 kg/mlsec, and more preferably from 200-350 kg/mlsec.

For cooling to a temperature not exceeding 1,200C it is as a rulesufiicient for the straight tube to have a length of about 2 meters. Ifit is desirable for the heat transmission to be increased, the gasvelocity may be increased and the tube length may be chosen longer thantwo meters to obtain a sufficiently long residence time. it is alsopossible to use several straight tubes arranged in parallel, eachconnected to a helical coilas defined.

If desired, the length of the straight tube may be chosen up to tenmeters. As a rule, however, this length will not be adopted on accountof the consequent height of the heat exchanger. For this reason, thetube length will preferably be kept smaller by using several straighttubes arranged in parallel, each connected to a helical coil.

it is preferred that at least some of the successive coils of thehelically coiled tube extend, at least substantially, in the directionof the straight tube. In connection with the space available, thelongitudinal axis of the coils may form a small angle with the extensionof the longitudinal axis of the straight tube. The connection of thestraight tube to the helically coiled tube may be such that thelongitudinal axis of the said coils is, at least substantially, in theextension of the longitudinal axis of the straight tube, or such thatthe longitudinal axis of the said coils is, at least substantially,parallel with the extension of the longitudinal axis of the straighttube. If desired, the helically coiled tube may consist of two parts,the arrangement being such that the first part extends in the directionof the straight tube and connects to a second part, the coils of whichhave the same longitudinal axis but have a different radius relative tothe longitudinal axis. This second part can be situated inside oroutside the first part, preferably on the inside. In this way concentrichelically coiled tubes are form ed.

At high steam pressures, for example of 80 atm. and higher, the lengthof the straight tube is preferably chosen larger than 2 meters, forexample 4-6 meters. The mass velocity in this case is preferably 200-350kglm lsec.

The cooling liquid is preferably introduced in such a way that thestraight tube (tubes) is (are) cooled in parallel flow with the gasesflowing in this tube (these tubes). During the cooling, at least part ofthe cooling liquid is evaporated and a mixture of coolant liquid andgenerated vapors formed. The same coolant also cools the helical coilswhere additional quantities of vapor (steam) are formed. It is generallyadvantageous (in view of the rate of flow and turbulence of the coolingmedium) to ensure that the free cross sectional area of the spaceaccommodating the straight tubes is not more than 30 percent of thecross sectional area of the space accommodating the helical coil(coils). In those cases where the abovementioned free cross sectionalarea is larger than 30 percent, use may be made of baffle platesprovided in the space accommodating the straight tube (tubes). Forexample, if four straight tubes are used, baffle plates having the shapeof a curved shield arranged symmetrically along the wall of the space,the concave side being turned towards the wall, are very suitable.

DESCRIPTION OF DRAWINGS AND PREFERRED EMBODIMENTS The invention will nowbe further explained with reference to the drawings in which differentembodiments of the invention are shown by way of example.

FIG. I is a diagrammatic representation of an ap* paratus for thepartial combustion of hydrocarbons and the cooling thereof.

FIG. II is a diagrammatic representation of an embodiment of the heatexchanger.

FIG. III shows a cross-section of an embodiment of the heat exchanger,through the space accommodating the straight tubes, and in which theheat exchanger is provided with four straight tubes, four helical coilsand with baffle plates which are arranged in the space accommodating thestraight tubes.

Referring to FIG. I, part A represents the actual reactor which isprovided with fuel supply line q leading to burner A of the reactor, andwith oxygen supply line b. If steam is used, it may be supplied througheither line q or line b. Part B is a connection between the reactor andconnecting piece C. The hot gases are passed through connection B andconnecting piece C into heat exchanger D comprising a vertical outershell including top and bottom closures which is provided with astraight tube and a helical coil, and further with discharge 0 for thecooled gases and an inlet and outlet for the coolant, d and e,respectively. The straight tube which has a length of at least 2 metersis designated by f, and the helical coil by g.

FIG. II is a partial longitudinal cross-section of an embodiment of theheat exchanger. The heat exchanger comprises a cylindrical vessell3.having a bottom plate 3, placed on a connecting piece 5, which isprovided with a gas supply line 4. The heat exchanger further comprisesdischarges 8 and 9 for the cooled gas, a coolant supply line 10, thebottom end of which is provided with a spray nozzle 11, helical coils 6and 7 connected to straight tubes 1 and 2, respectively, the length ofwhich is at least 2 meters. The coolant, preferably water, is suppliedthrough the line 10 and is sprayed against the bottom plate subsequentlyflowing upwards, thereby cooling straight tubes 1 and 2 and helicalcoils 6 and 7. The helical coils are arranged in annular space 14 formedby the wall of the supply line and the shell of the cylindrical vessel.The helical coils have a common longitudinal axis which coincides withthe longitudinal axis of the supply line. The heat exchanger further hastwo baffle plates for the cooling water which extend from the bottomplate to substantially the place where the helical coils connect to thestraight tubes. The location of these baffle plates is not shown.

In operation, a hot-soot containing gas at a temperature of 1,300 to1,400C or higher, e.g., crude synthesis gas, is introduced intoconnecting piece 5 via gas supply line 4. The hot gas is flowed throughstraight tubes 1 and 2 at a mass velocity of at least kglm lsec. The gasin the straight tubes is cooled to a temperature between l,000l,200CC bymeans of a coolant liquid, in this case water, supplied through line 10and sprayed against bottom plate 3 by means of spray nozzle 11. Uponstriking the bottom plate, the water flows upward in a substantiallyparallel direction to the flow of gas in tubes 1 and 2, cooling both thestraight and helically coiled tubes. Steam generated by the partialvaporization of the water in contact with the outside walls of thetubes, ascends with the remaining liquid coolant and is dischargedthrough line 12. The cooled gas, e.g., at a final temperature of about200C to 400C is discharged through lines 8 and 9. By operating in thismanner it is possible to generate steam at pressures of from 50 toatmospheres or higher without subjecting the helically coiled tubes toexcessive pressure differentials and without experiencing anysubstantial soot deposition problems.

FIG. III is a cross-section through the space accommodating the straighttubes of an embodiment of a heat exchanger having the configurationshown in FIG. II, but which has four helical coils connected to fourstraight tubes. The cross-section shows the baffle plates for thecoolant, the four straight tubes and the coolant supply line. In thedrawing the reference numerals 20, 21, 22 and 23'designate the straighttubes, 24 is the coolant supply line, 25 is the shell of the heatexchanger, 26 is the space accommodating the tubes 20-23, and 27, 28, 29and 30 are shield-shaped baffle plates for the coolant, which aresecured to the shell 25.

WE CLAIM AS OUR INVENTION:

1. In a process for the preparation of synthesis gas by the partialcombustion of hydrocarbons using oxygen or oxygeneenriched air whereinsaid synthesis gas is cooled in a helical coil waste heat boiler, theimprovement which comprises generating steam in said waste heat boilerat a pressure of 50 to 150 atmospheres from the sensible heat containedin said gas, by flowing said gas at a mass velocity of from 100-500 kg/m/sec through a straight tube of 2-10 meters in length which is inexternal contact with water thereby cooling the gas to a temperaturebetween 1,000 and 1,200C, and subsequently passing said gas through ahelically coiled tube which is also in contact with water, saidhelically coiled tube being connected to said straight tube.

2. The process of claim 1 wherein the water is in substantially parallelflow with the hot gases flowing in the straight tube.

3. The process of claim 1 wherein the straight tube has a length of from4-6 meters.

4. The process of claim 3 wherein the gas is flowed through the straighttube at a mass velocity of from 200-350 kg/mlsec.

2. The process of claim 1 wherein the water is in substantially parallelflow with the hot gases flowing in the straight tube.
 3. The process ofclaim 1 wherein the straight tube has a length of from 4-6 meters. 4.The process of claim 3 wherein the gas is flowed through the straighttube at a mass velocity of from 200-350 kg/m2/sec.